The present invention relates to chromate conversion coatings and, more particularly, to compositions and processes for producing chromate conversion coatings on metal surfaces so as to afford corrosion resistance to the metal surfaces.
Chromate conversion coatings are well-known in the metal finishing art and have long been used to provide corrosion resistance to articles either made from, or coated with, metals such as zinc, cadmium, zinc-cobalt alloys, and the like, and also may serve to provide a desired decorative finish to the article. The coatings are referred to as conversion coatings because the treatment brings about reaction at the metal surface so as to convert the metal surface to a superficial layer containing a complex mixture of chromium compounds.
Chromate conversion coatings can, by variation in composition of the solution from which they are applied and to some extent variation in the conditions of treatment, be applied to the metal surface in a variety of coating (or film) thicknesses, with the degree of corrosion resistance afforded being generally proportional to the coating thickness. Generally speaking, the chromate coating is very thin, ranging, for example, from about one to about 50 microinches. As is well-known in the art, the generally thin nature of these coatings, coupled with the generally increasing content of chromium associated with increasing coating thickness, enables the chromate coatings of varying thickness to be broadly characterized on the basis of color. Thus, for example, where the metal surface is zinc, cadmium or zinc-cobalt alloy, chromate conversion coatings, from thinnest to thickest, can be broadly characterized as blue-bright, iridescent yellow, brown, bronze, olive-drab and black.
The essential components of a composition used to treat a metal surface for provision of a chromate conversion coating thereon are, in aqueous solution, a source of hexavalent and/or trivalent chromium ions, acid, and a source or sources of so-called activator anions, generally from among the group of acetate, formate, sulfate, chloride, fluoride, nitrate, phosphate and sulfamate ions, which generally determine the character, rate of formation and properties of the chromate film, all as well known in the art. Generally speaking, the formation of chromate conversion coatings on metal surfaces involves immersion of the article in the composition for the time required to produce the desired coating, although treatments involving spraying, brushing, swabbing or even electrolytic techniques are also known and used.
A general aim of research and development efforts in chromate conversion coatings is to provide compositions and coatings which will afford the best possible corrosion protection to the metal surface in the thinnest possible coating, since in this way increased costeffectiveness can be obtained and, in situations where particular coating color is desired, improved corrosion protection in that coating color can be obtained. Also, it is known that for particular metal surfaces, certain chromate coatings of desired color are not acceptably attainable. This is the case, for example, for iridescent yellow chromate coatings on zinc-cobalt alloy surfaces, where prior art yellow chromate coating compositions generally result instead in grayish films (dark gray recesses on the zinc-cobalt surface).